1. Field of the Invention
The present invention relates to a technique of effecting a signal processing in an optical gyro. More particularly, it relates to a signal processing apparatus for an optical gyro in which light having a constant wavelength is propagated simultaneously clockwise and counterclockwise through an optical propagation path such as an optical fiber in cooperation with a rotation axis and receives a phase modulation and then a phase difference of the light based on Sagnac effect is detected to thereby obtain a signal proportional to an input rotation angular velocity or an angular increment signal.
2. Description of the Related Art
A typical fiber optical gyro using a phase modulation includes a propagation path of an optical fiber through which light is propagated simultaneously clockwise and counterclockwise, an optical system optically coupled to the optical propagation path and having a phase modulator for giving a phase modulation to the propagated light, and a signal processing apparatus for effecting an analog demodulation of a photoelectric output signal obtained from the optical system and thus driving the phase modulator. In a known example, the signal processing apparatus includes a number of analog circuits, e.g., an analog multiplier, a low pass filter, a band pass filter, an oscillator, and the like, which respond to the photoelectric output signal and generate a drive signal for the phase modulator.
As described later in detail, however, the driving technique employing such an analog demodulation has a number of drawbacks. For example, where offset voltages occurring in each analog circuit fluctuate, a bias stability of the gyro is lowered. Also, even if the offset voltages are zero, the gyro output or angular increment output is proportional to sin .phi. s (Note, .phi. s is a phase difference of light based on Sagnac effect) and thus a problem occurs in that a linearity of the gyro for the input rotation angular velocity is not good
Also, a drawback occurs in that, when a navigation object loaded with the fiber optical gyro is in movement, a detectable maximum angular velocity is limited to a narrow range. Furthermore, another problem occurs in that the linearity and scale factor stability of the gyro output are easily deteriorated due to a fluctuation in a quantity of incoherent light of a light beam in the optical system, a fluctuation in a phase modulation index of the phase modulator, or the like.
Note, the problems in the prior art will be explained later in detail in contrast with the preferred embodiments of the present invention.